The i3 6100 and the a10 7870k are both "Quad Core APUs" in the sense that both offer 4 Logical Cores and an OpenCL/Video Accelerator to your system. That is, the i3 6100 has hyper-threading. Because the Intel chip has hyper-threading it offers not only better single-threaded performance but multi-threaded performance as well. Sorry AMD but as a developer I don't consider CMT modules to be two whole cores.
As of Jan 2016, the one thing APUs and iGPUs offer gaming is hardware support for video encoding for streamers. Intel in my opinion does this better, as you can stream to Twitch using Intel Quicksync hardware acceleration to encode the video with almost zero performance penalty. The a10 APU can encode the video using the shaders in the APU using OpenCL, which is useful if you're encoding in something other than HVEC or h264, tho the Skylake iGPUs make for half decent OpenCL accelerators too.
To be honest, the only thing the a10 offers is a better OpenCL accelerator, and unless your streaming, for gaming workloads this doesn't even matter because no games use it. In video and photo editing type workloads where the APU might be used, you're probably better off using a real GPU anyway. I would suggest pairing anything from a 4GB 960/380 to a 970 or 390 with this system, and going SLI/Crossfire in the next generation or two when prices drop, tho I'd suggest getting an 8320e FX if and 970FX board instead if you're going with a discreet card.
As for those motherboards, they are poor choices. I'm sorry but the Asrock Killer board is all bling, and the MSI board looks the same. They are not even good for gaming as they don't support SLI, they're literally just bling. They do have good build quality, but so do all Asrock, Asus or Gigabyte boards....
i3 6100 board....
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157675&cm_re=z170_extreme_4-_-13-157-675-_-Product
a10 7870k board...
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157457&cm_re=a88x_atx-_-13-157-457-_-Product
Both of these boards should work in both Crossfire or SLI. One does Gen3 8x/8x/4x the other does Gen3 8x/8x/Gen2 4x, and that 4x would still be plenty useful for something like an SSD or USB3.0 or USB3.1 IO card say if you needed the USB3.1 ports for an Occulus Rift. Or as a PhysX card if you went with SLI.
Honestly though, if you have a video card get an 8320e and a 970 FX board, cheaper than an APU, faster than an APU, SLI support, support for overclocking (def get a Hyper 212 Evo later), 1866 memory (sweet spot for FX) is cheap enough,....
http://www.newegg.com/Product/Product.aspx?Item=N82E16813132713&cm_re=970_fx-_-13-132-713-_-Product
The 8 core CPU from AMD are pretty solid and compete very well against the i5, and 970 FX boards are more than enough for even high end gaming rigs. The main feature of 990FX is quad-crossfire or quad-sli, which is a very niche space, and while the 8320e with a 212 evo and modest overclock isn't going to trump an i7 it will deliver mid-range performance (sometimes better, sometimes worse than an i5) at a very low price point.
PS: Yes even a Pentum Dual core will beat it in single threaded workloads, like.... Quake1 or Duke Nukem Atomic Edition. Don't worry though, all major modern 3d engiens are becoming more and more threaded and this trend shows no signs of slowing down or changing course back to how things were, in fact, it's still growing.